1. Field of the Invention
This invention relates to molecular beam epitaxial methods and apparatus for epitaxial growth of a compound upon a substrate.
2. Description of the Related Art
The present invention is concerned in general with the growth of structures by molecular beam epitaxy (MBE), and in particular with the use of MBE for the epitaxial growth of mercury telluride (HgTe), cadmium telluride (CdTe) and mercury cadmium telluride (HgCdTe) single crystal alloys, and HgTe/HgCdTe superlattices. HgCdTe is difficult to prepare for use in detection devices by either bulk or epitaxial techniques. The most commonly used epitaxial growth process for these materials is currently liquid phase epitaxy. Although high performance infrared detectors have been realized with growth by liquid phase epitaxy, the technique cannot produce abrupt heterojunctions and superlattices required for advanced opto-electronic devices. A review of various growth techniques is provided in J.P. Faurie et al., "Latest Developments in the Growth of Hg.sub.l-x Cd.sub.x Te and CdTe-HgTe Superlattices by Molecular Beam Epitaxy", J. Vac. Sci. Technol. A, Vol. 1, No. 3, Jul.-Sep. 1983, pages 1593-97.
The MBE technique, on the other hand, is suitable for the growth of high quality epilayers, abrupt heterojunctions and alternate microstructures such as superlattices. This technique is described in J.P. Faurie et al., "Molecular Beam Epitaxy of II-VI Compounds: Hg.sub.l-x Cd.sub.x Te", J. Cryst. Growth, Vol. 54, No. 3, pages 582-85, 1981. However, the growth of HgCdTe by MBE is hard to control because of the excessive mercury re-evaporation from the surface during the deposition process.
MBE is a vacuum deposition process. The current implementation of the process uses several effusion cells, each cell comprising an electrically heated crucible containing one of the substances of the compound to be grown. Upon heating, the cells produce atomic or molecular beam fluxes of mercury, cadmium and tellurium. The fluxes are directed onto the surface of the substrate, where they react with each other and produce an epitaxial layer.
The growth rate of an MBE process is critically dependent upon the "sticking coefficient" of the materials being grown, i.e., the probability that a particle of the flux will adhere to the surface of the substrate. In the case of HgCdTe and HgTe/HgCdTe superlattice growth, the Hg sticking coefficient is very low. For a substrate temperature range of 170.degree.-200.degree. C., the Hg sticking coefficient has been found to vary between about 10.sup.-4 and 10.sup.-3. With conventional MBE growth, therefore, large Hg fluxes must be used. For example, as described in J.P. Faurie et al., "Molecular Beam Epitaxy of Alloys and Superlattices Involving Mercury", J. Vac. Sci. Technol., A3(1), 1985, pages 55-59, kg of mercury is required to grow a 75 micron thick layer of Hg.sub.l-x Cd.sub.x Te. This is an undesirably high rate of mercury consumption, and also requires a relatively high substrate temperature. Furthermore, it is difficult to control the electrical properties and to attain abrupt junctions for heterostructures.